I have a totally crazy idea, feel free to shoot it down.The idea is a Tri-propellant (but not really) MCT.Assuming that:1. Methalox engines are required for Mars-side ISRU.2. Most payload mass will only go one way: Earth to Mars.How much sense would it make to use Hydrolox engines for the Mars-bound half of the trip?Basically, you use highly efficient Hydrolox engines to get to Mars, use ISRU there to fill the hydrogen tank with methane, and use Raptors to go back home (and I know, the tank volume ratios are problematic).You may say, but SpaceX doesn't have Hydrolox engines. True, but Blue Origin does. :-)
You'd actually get much more performance by filling the tanks with methane than with hydrogen.Hydrogen has terrible density, so you could fit a LOT more methane in there than hydrogen.
Quote from: Robotbeat on 08/26/2016 02:58 pmYou'd actually get much more performance by filling the tanks with methane than with hydrogen.Hydrogen has terrible density, so you could fit a LOT more methane in there than hydrogen.This doesn't make sense. There is a reason Saturn 5, STS, SLS and the Centaur all use hydrolox...
Hydrogen already has a lot of negative design trades, and if you add in the requirement that the system also has to burn methane equally well, there's no good reason to use hydrogen at all.And the tank volume ratios are more than slightly problematic. The hydrogen tank to put 180 tonnes through TMI and EDL would be more than 7 times larger in volume than the methane tank needed to return a 80 tonne vehicle from Mars surface to Earth's surface. It would be larger in volume than most estimates for the entire BFR booster.
Assuming that:1. Methalox engines are required for utilizing Mars-side ISRU.2. Most payload mass will only go one way: Earth to Mars.
Quote from: Eerie on 08/26/2016 02:57 pmAssuming that:1. Methalox engines are required for utilizing Mars-side ISRU.2. Most payload mass will only go one way: Earth to Mars.False assumption.Mars ISRU is not capturing existing methane as we do on earth. Mars ISRU envisions building the methane from water derived H2 and atmospheric CO2. If the system had H2 engines and tanks you'd refill them with H2 and skip the methane altogether.In other words, if the trades worked in hydrogen's favor, there's nothing preventing them from building a hydrogen rocket without the methane. Since they have chosen to build a methane rocket, one might presume that the trades did not work in hydrogen's favor. Quite possibly for some of the reasons mentioned above.
Thanks for demolishing my proposal. :-)So, I am correct to understand that the increased mass of the tanks would eat all the performance gains from using Hydrolox?
Quote from: Impaler on 08/26/2016 01:38 amLastly some estimates on mass, as noted earlier the leg system on F9 masses 2500 kg, carbon fiber fabric 5 layers thick totaling 1 kg/m^2 and covering the space between a 15 m base and a 51 m total diameter would be 1800 m and kg bringing total mass to 4.3 mt, a very modest amount that would be far lighter then carrying extra propellant. The propellant needed for just the last bit of deceleration and up to landing is going to need to do around 800 m/s based on extrapolation from Red Dragon, and that's going to come out to around 20 percent propellant fraction minimum. I'm estimating 200 mt at entry and 40 mt of landing propellant.Reducing dry mass will have priority since the architecture is constrained by the return leg. The main benefit of something like HIAD would be its lower mass compared to a mid L/D aeroshell.
Lastly some estimates on mass, as noted earlier the leg system on F9 masses 2500 kg, carbon fiber fabric 5 layers thick totaling 1 kg/m^2 and covering the space between a 15 m base and a 51 m total diameter would be 1800 m and kg bringing total mass to 4.3 mt, a very modest amount that would be far lighter then carrying extra propellant. The propellant needed for just the last bit of deceleration and up to landing is going to need to do around 800 m/s based on extrapolation from Red Dragon, and that's going to come out to around 20 percent propellant fraction minimum. I'm estimating 200 mt at entry and 40 mt of landing propellant.
Regarding decelerator devices...Isn't it somewhat counter-intuitive to put them at the front of the vehicle? Wouldn't such a vehicle normally flip around to have the area of maximum resistance at the back?I am thinking that any large-scale decelerators should be deployed at the rear of the vehicle, as a form of drogue chute. That would give a heatshield of vehicle diameter at the front and a large-diameter deployable decelerator trailing behind, to create a stable configuration. And no, I am not just talking about a parachute, because I think of a much more robust device, be it with fins or petals or whatever the aerodynamicists can come up with.
Approaching 30 days from what (we hope) is the big reveal, I thought it a good time to revisit and post revised BFR/MCT speculation before any info leaks out. Trying to stay within the parameters of what Musk has said as I best understand. A TSTO vehicle launched by a re-useable, single core BFR that puts the BFS a.k.a. the MCT into LEO where it is re-fueled, travels to and lands on Mars where it is again refueled for the journey back to Earth carrying a quarter of the outbound “cargo” mass. The outbound cargo masses 100 tonnes which I assume means either cargo or people or a combination thereof. BFS/MCT mass not included in the 100T.Myriad unknowns led by the dry mass of the BFS. Rocket equation dictates various mass assumptions here can produce wildly different answers.My predictions, metric unless otherwise stated:1.Entire launch vehicle BFR+BFS masses under 5,000T. Guestimate ~4,500T.2.BFS dry mass < 100T, my pick is 85T carbon composites BUT heavier than some predictions because ruggedized to allow for minimal maintenance.3.BFR absolutely > 10m diameter to fit enough engines. Likely between 12.5 and 15m. My guess 15m. Allows addition of more engines in the future.4.My guestimate BFR+BFS stack <100m height. Certainly <125m.5.Sticking with the “over 230T” Raptor thrust Elon mentioned, I get 25-27 engines. My guestimate is 26 with “over 230T” as 235T in my spreadsheet. Around 13.5 million Lbs force.Engine # most likely wrong because…6.Predict that Raptor engine design goal thrust changed to higher than 230T previously stated, but only by several 10s of tonnes, not hundreds.7.BFS with 5 Rvac engines8.RTLS minimizes cost, turnaround time, effort. Changed my opinion from max payload ASDS for those reasons. Just make the BFR bigger. Stages low and slow ~2.2 Km/sec. “Easy” recovery & re-flight vs F9 GTO flights.9.Initial BFR test flights likely equipped with less engines and less payload.10.Large crew volume design >2,000m3. Initial flights with less people & people space but more cargo space.11.Initial crewed Mars mission will carry 6-12 people. 10 is my latest #. Why? NASA & other nations will buy seats. http://forum.nasaspaceflight.com/index.php?topic=40683.msg1557261#msg155726112.SEP still under development awaits later opposition cargo transits 13.BFS will have “exotic” upper mounted engines for rough terrain Mars landing &takeoff (just echoing others’ analysis here)14.BFS will be a lifting body for EDL, but not a scaled up Dragon capsule shape. It will look badass.You know we’re totally screwed trying to predict Musk because he already warned us, “When it looks more like an alien dreadnought, that’s when you know you’ve won.”I’ve attached a spreadsheet showing different assumptions, BFS mass, etc.Anyone else want to update their speculations?
As for exotic upper mounted engines, I used to think they were required, but now I think terminal landing will be done on raptors alone (again to save dry mass). For this to be possible from the first BFS landing will require scouting an appropriate flat solid rock surface and a dragon placed rover to sweep away any rocks/pebbles and place beacons to allow for guidance to the landing pad within a few meters.
Whilst I agree that Raptors alone would save dry mass, as well as removing the dependency on two successfully serialised miracles, if a Red Dragon could safely land on an unprepared Martian surface, why couldn't a BFS, especially if it resembles a scaled up Dragon?